Saw-tooth wave generator



March 7, 1939. H, HORE ET A 2,149,471

SAW-TOOTH WAVE GENERATOR Original Filed May 26, 1934 OUTPUT- I: & kinill I Q I v INVENTOR k N MFA/R) J/lO/PE g JAMES W mks/e BY/k ATTORNEY Patented Mar. 7, 1939 UNITED STATES SAW-TOOTH WAVE GENERATOR Henry Shore, Brooklyn,

and James N. Whitaker,

Tuckahoe, N. Y., assignors. to Radio Corporation of America, a corporation of Delaware Application May 26, 1934, Serial No. 727,688 Renewed March 19, 1937 8 Claims.

The present invention relates to a method and apparatus by which electrical energy of one type of wave formation can be converted into electrical energy of a different type wave formation either of the same or different frequency but bearing a harmonic relationship to each other.

In the transmission of intelligence and preferably in the form of a picture record where an electro-optical image or likeness is to be produced at a receiving point, it has been customary as explained in our copending application Serial No. 558,750, filed August 22, 1931, entitled Picture reproducing apparatus which issued as U. S. Patent No. 2,083,245 on June 8, 1937, to develop what we term a screen tone to serve as a background from which the effects of the reproduced intensities of light and shadow in the electro-optically reproduced picture will approximate very closely the well known half tone. In such systems the record or other subject of which the electro-optical reproduction is desired is scanned in the usual manner, element for element, and a resulting electrical current is produced which corresponds successively to each individual element of the subject. It is customary in scanning to impress upon the scanning element a tone frequency which can be modulated by the varying intensities of light and shadow on elemental sections of the picture record.

According to the system explained in the above mentioned copending application, the picture modulated tone frequency so developed is rectified and supplied to a mixing tube. Simultaneously with the application of the rectified picture modulated tone frequency there is impressed upon the mixing tube the screen frequency which can be of any desired value. In such a system the mixing tube is preferably so biased that the screen tone does not actuate the grid throughout its entire cycle, that is, for example, when the output of the rectifier represents the white portion of the picture the screen tone is completely blocked from passing through, whereas for a black the complete cycle of the screen tone can actuate the grid of the mixing tube, and for a middle gray, for example, but half the cycle of the screen tone actuates the grid of the mixing tube. Connected to the output of the mixing tube it is customary to use What we term a square wave amplifying device to convert the output energy from the mixing tube into constant amplitude impulses of varying width wherein the width depends upon the amplitude of the mixing tube out- .put.

For transmission in accordance with the above outlined system, the output energy from the square wave amplifying device is used to key a tone or a carrier frequency which, in turn, serves to key a radio transmitter or a wire line distribution system. While the square wave amplifying device is disclosed in our above mentioned copending application, it is also disclosed more particularly in copending application of Henry Shore, Serial No. 528,796, filed April 9, 1931, Patent No. 2,005,111, granted June 18, 1935.

The present invention improves upon that described in our above mentioned copending application by improving the stability of operation and avoiding all distortion which may be due to the use of sinusoidal shaped screen tone impulses. Such distortion, which is most severe at the extreme ends of the density range, would cause an abrupt change at either end of the scale, for example, from pure white to light gray at one end and from dark gray to pure black at the other end. In view of the fact that it is undesirable to have abrupt changes in density the range for satisfactory operation with sine wave screen frequency input may be considered, for example, as being within the range of ten percent to ninety percent marking. This reduction in the density ratio tends to produce what might be considered as a flat picture and consequently militates against most satisfactory electro-optical reproduction to the transmitted picture or subject.

Therefore, by the present invention We have sought to provide ways and means for developing a screen frequency input to the mixing tube which is in the form of an isoceles triangle wave shape which will provide smooth control from zero to one hundred percent marking because the duration is exactly proportional to the amplitude and further because no distortion is produced during any portion of the range and consequently truly linear system of transmission and reception is provided. Such a wave formation has been commonly termed a saw-tooth wave and the particular formation of saw-tooth wave generator which is herein disclosed is one which is symmetrical in all respects, that is, the rate of change from zero to maximum corresponds with the rate of change from maximum to zero.

Therefore, as a primary object of our invention, we have sought to develop awave generator primarily for use as a screen tone generator which will improve substantially the operation of a picture transmitting and receiving system.

A further object of our invention is to provide ways and means for developing electrical Wave formations which are suitable for controlling the beam in a cathode ray tube using either electromagnetic or electrostatic deflection or a combination of both where such tubes are used either for oscillograph or television purposes.

Still further objects of our invention are to provide a wave shaping system capable of producing electrical waves of desired formation which bear a harmonic relationship, that is, which are of a frequency above or below some particular controlling frequency having an electrical wave the art to which the invention relates and will,

of course, become apparent from a reading of the following specification when considered in connection with the accompanying drawing wherein the single figure thereof forms schematically one form of apparatus capable of carrying out the aims and objects of this invention.

If reference is now made to the accompanying drawing, it will be seen that the unit herein to be described comprises 'a source of input frequency which is so directed as to energize a multi' vibrator which, in turn, controls a wave shaping tube so as to energize a circuit which converts the approximately sinusoidal input energy into the desired form of saw-tooth electrical wave. As herein disclosed, it is assumed that the frequencies being dealt with are of audible or superaudible range and, therefore, transformer coupling has been illustrated although, of course, re-

sistance or impedance couplings could be substituted where desired.

If reference is now made to the specific construction shown in the drawing, sinusoidal waves of some selected or chosen frequency and controlled ,as to stability are impressed across the input terminals l and 2 and fed by way of the transformer 3 to the circuit including the resistor element 4 grounded at 5. A second resistor element 6 is arranged with a plurality of taps I V with which a contact arm 8 is adapted to contact, the contact arm in turn connecting at a point 9 to selected points onthe resistor 4 so as to control the amplitude of the input signal impressed upon the grid electrode ID of the thermionic tube II. The thermionic tube H is preferably of the type known in the art as the twin amplifier and commonly referred to as the RCA- 79, although it is to be understood that a plurality of three-element tubes might 'be substituted where desired in place of using a twin tube contained within one envelope. The tube H comprises the heater element 12, the cathode !3, the pair of grids l0 and M and the plate electrodes 55 and I6. To form the multi-vibrator unit, the

grid and plate electrodes of the tube II are interconnected so that the grid electrode II], for example, connects with the plate electrode [6 by way of the capacity !8, and the grid electrode l4 connects with the plate electrode I5 by way of the capacity [7. Connection between each plate electrode and a source of voltage to energize the plates are'a pair of resistor elements l9 and 20 connecting by way of the choke 65 to the battery 63 serving as the plate voltage supply. One terminal of the battery is preferably grounded at B l although this is not essential where more than a single wire systemis desired.

The multi-vibrator unit comprising the tube I I and its connections above specified is so arranged A that connected to one output circuit thereof is a resistor element 2| having a plurality of taps 22 with which the switch arm 23 makes contact. The switch arms 8 and 23 are arranged by way of the connection 25 to operate in unison and the resistors 6 and 2| are preferably non-symmetrical. By adjusting the contacts 8 and 23 the frequency of the multi-vibrator unit may be controlled. In varying the frequency of the multivibrator the resistors 6 and 2| are so arranged that the larger the value of the various steps I which are tapped and brought to contact with the contact arms 8 and 23 of the selector switch,

the lower will be the frequency developed by the multi-vibrator. The uncontrolled grid M of the multi-vibrator is arranged to connect with one grid electrode 21 of a tube 28 which is also preferably of the twin amplifier type, although as was suggested in connection with the tube II, a plurality of separate amplifying tubes may be substituted.

The tube 28 comprising the gride electrodes 2! and 33, the heater element 29, the cathode 3! and the plate electrodes 34 and 35 serves the dual iunctionof producing a square sided Wave and the saw-tooth wave signals brought to the grid 21 from the uncontrolled grid IQ of the multivibrator H serve to control the current flowing in the output circuit including the plate 34 of this tube. When the first grid 21 of the tube 28 is blocked sufficient voltage is applied to the glow tube 3? connected in the output circuit of this tube to cause it to break down and to'draw current through the resistor 39. When the potential drop across the resistor 2! is removed on the next half cycle the first grid 2'! will be unblocked and current will flow through the first plate 34 and the resistor 4|. The voltage drop through theresistor 4| is so great that insufficient voltage to maintain the glow lamp remains with the result that the glow lamp 3'! is'extinguished and no current flows through the resistor 39. Because of the characteristics of the glow lamp 31, that is, the sudden break down and extinction, the potentialdrop across the resistor 39 which is applied to the second grid electrode 33 of the tube 28 are square sided and square topped. The current which flows through the second plate electrode 35 of this tube and the impedance network 43 including the condenser 45 and the primary winding of the transformer 41 is therefore in the form of an impulse. This current flow induces in the secondary winding of the transformer 41 V a saw-tooth wave which is then amplified by any desired form of conventional high quality transformer coupled amplifier. To regulate the amplitude of the signal applied to the control grid 7 d9 of the amplifie'r'tube 5! there is provided a resistor 53 towhich an adjustable connection from the grid may be made. the secondary of the transformer is connected to ground at 50 and there is provided'a connection to the cathode 54 which is heated by the heater element 55 and connection to ground by way of the resistor 56 and the condenser 51. The plate electrode 59 is adapted to feed the output energy through the primary'of the transformer 60 from which it is transferred to .the output circuit and the terminal points 6! and 62.

As has been shown herein, all tubes are provided with a common source of heating current conventionally designated by the battery 58 having one terminal connected to ground and one terminal of all heater elements also connects to ground. Platevoltages for all tubesare supplied from a common supply 63 grounded at one terminal at 64 and by means of the chokes 65 and ,6! ripplesare suitably smoothed out and One terminal of the voltage regulator in order that the output voltage shall at all times remain constant.

Many other modifications and changes, of

. course, may be made in the system without de-- parting in any way from the spirit and scope of the invention as described herein and defined in the hereinafter appended claims.

Having thus described our invention, what we claim and desire to secure by Letters Patent of the United States, is the following:

1. A wave shaping system comprising a source of substantially constant frequency input current, a two stage capacity coupled thermionic amplifier having a portion of the output energy coupled back to the input thereof, a connection between the input of said amplifier and the source of input current, a second two-stage amplifier connected in cascade with said first two-stage amplifier and having its first input circuit controlled from the output circuit of one portion of said first two stage amplifier, a glow discharge device connected in shunt with one output circuit of the first stage of the second cascade amplifier and an impedance network connected in the output circuit of the second stage of the second cascade amplifier whereby a current flows alternately through the glow discharge device and the impedance network during time periods determined by maximum and minimum values of the output from the two stage amplifier supplied to the eascade amplifier, and an additional amplifying device having its input circuit energized by the current in the impedance network so that the output current thereof rises and falls along linear paths.

2. A circuit for developing symmetrical sawtooth electrical waves from sinusoidal waves which comprises a source of constant frequency sinusoidal alternating current energy, a twostage capacity-coupled thermionic amplifier having a portion of its output energy coupled to the input thereof, a second two-stage cascade connected thermionic amplifier tube having one of its input circuits connected with the output of one portion of said first-named cascade amplifier, a variable impedance device connected across one output circuit of said second cascade amplifier whereby current flows through said variable impedance device only during time periods when output current ceases to flow in said output circuit, and the first stage of the said cascade amplifier serves as a first wave shaping device, a second wave shaping device comprising the second half of said second cascade amplifier connected in the circuit including the said variable impedance device so that current flows therethrough during periods of operation of the said variable impedance device, an impedance network connected with the output of said second wave shaping device, and an amplifier having its input circuit connected with said impedance network whereby the output current therethrough rises and falls along symmetrical linear paths.

3. A circuit claimed in claim 2 wherein said variable impedance comprises a glow discharge device.

4. A circuit claimed in claim 2 comprising in addition a pair of unequal resistor elements connected across the grid circuit of each input portion of said first-named two stage capacity coupled amplifier for controlling frequency of the output with respect to the frequency of the supplied energy.

5. A wave shaping system comprising means for producing rectangular wave energy of variable frequency, means for supplying wave control energy of a stable predetermined frequency, means for controlling the frequency of the rectangular wave energy by the supplied energy and means including a parallelly connected capacity and inductance having inductive reactance at the frequency of the rectangular wave for converting the controlled rectangular wave energy into wave energy having isosceles triangular form.

6. A wave shaping system comprising means for producing rectangular wave energy of variable frequency, means for supplying wave control energy of astable predeterminedfrequency, means for controlling the frequency of the rectangular wave energy by the supplied energy, means including a parallelly connected capacity and inductance having inductive reactance at the frequency of the rectangular wave for converting the controlled rectangular wave energy into Wave energy having isosceles triangular form, means for amplifying the isosceles wave form energy, and an output circuit connected to said amplifying means.

7. In a wave shaping system including a parallelly connected capacity and inductance having inductive reactance at a predetermined frequency, the method of producing a symmetrical saw-tooth wave which comprises the steps of producing rectangular wave energy of said predetermined frequency supplying control wave energy of a stable frequency, controlling the frequency of the rectangular wave energy by the supplied energy to stabilize the frequency of the rectangular wave energy at the frequency of the predetermined frequency, and supplying the controlled rectangular wave energy to the parallelly connected capacity and inductance, whereby the rectangular wave energy is converted into wave energy of isosceles triangular form.

8. In a wave shaping system including a parallelly connected capacity and inductance having inductive reactance at a predetermined frequency, the method of producing a symmetrical saw-tooth wave which comprises the steps of producing wave energy having substantially perpendicular wave front and tail of the predetermined frequency, then limiting both the maximum and minimum values of the produced energy to predetermined values of amplitude, supplying wave control energy of a stable frequency, controlling the frequency of the limited energy by the supplied energy to stabilize the frequency of the limited energy at the frequency of the predetermined frequency, supplying the controlled limited energy to the parallelly connected capacity and inductance whereby the controlled limited energy is converted into wave energy of isosceles triangular form, and amplifying the converted isosceles triangular form wave energy.

HENRY SHORE. JAMES N. WHITAKER. 

